The invention relates to self-supported profile wire flat deck screen panels mounted on a screen support assembly to dewater and classify materials, and particularly to arrangements for mounting said panels so that they can be used in a vibrating apparatus. The screen panels typically comprise a plurality of parallel stainless steel wires which are each welded to an underlying series of spaced apart support rods or bars positioned normal to the wires. A principal use for such screen deck panels is in taconite processing plants, where the screen panels are used to help separate taconite ore from silicates and carbonates.
Taconite is a fine grained sedimentary rock of magnetite, hematite and quartz, mined as a low-grade iron ore. The taconite is a very hard rock that possesses a small amount (about 21%) of magnetic iron. The process and equipment used to extract the iron from the rock is quite complicated since the rock is a solid mass, much like a solidified lava flow. This rock must be blasted into chunks, transported from the mine to the processing plant and crushed to the consistency of face powder before the magnetic particles of iron can be separated from the waste rock. The rock is crushed to near size and is then mixed with water and transported as a slurry through piping to a screen box assembly. The slurry flows over the screen surface, which is typically positioned at about a 45.degree.-60.degree. angle to the horizontal, and the properly sized material which passes through the slots in the screen is then transported as a slurry to large magnetic rotating drums. The magnetic particles of iron are attached to the drums while the non-magnetic waste material continues on to waste sediment ponds. The magnetic particles are scraped off the rotating drums and again mixed with water and transported as a slurry to a further processing stage. Any oversized material which cannot pass through the screen slots continues to flow over the top surface of the screen and is returned to the crushers for further size reduction. Because the slot openings in the screens are quite fine, typically 0.003"-0.006", they are prone to blinding or plugging due to surface tension and the consistency of the taconite slurry. When the slots get plugged, the material which is of a small enough size to ordinarily pass through the slots cannot pass through them and thus remains mixed in with the oversized particles that flow over the screen surface. Since the material that does not pass through the slots is returned to the crushers for reprocessing, it is obvious that production is lost and energy is wasted when screen sized particles do not pass through the slots. To overcome the aforesaid plugging or blinding which greatly reduces the efficiency of the screening operation, it has been found to be necessary to rap or vibrate the screen at timed intervals. The vibration clears the slot openings, but has created other problems. The main problem is that the wear life of the screen panel and/or the screen box assembly which supports it, is greatly reduced. This is a result of the fact that prior art constructions typically require that the screen and its framing be welded together and that a vibrator mounting plate then be welded to the rods of the screen. A vibrator or rapper is then bolted to this assembly. The vibrations which are produced can result in an unpredictable wear life for the assembly. Although it is desirable that the wear life of a screen panel be determined by the time it takes for abrasion of the particles passing through it to widen the slots defined by the screen wires to an unacceptable dimension, the typical failure mode is breakage at the weld joints. Since such breakage renders the assembly useless, the entire assembly has to be discarded, resulting in an unacceptable economic loss. Wear is also greatly increased by the fact that, typically, a tremendous quantity of very hard taconite is processed, such as more than 100,000 tons per day.
Similar screen systems are used in the coal industry except that in the coal process, the fines which pass through the slot openings go to waste while the oversize material is the useful product that continues on for further processing.
There are a number of U.S. patents which appear to disclose structure wherein there is direct contact of a vibrating metal screen member by an element having an elastomeric contact surface, including: Kempa U.S. Pat. No. 3,929,647; Deister et al U.S. Pat. No. 4,137,157; Wilson U.S. Pat. No. 4,288,320; Heilhecker U.S. Pat. No. 4,341,627; Johnson et al U.S. Pat. No. 4,529,510; and Connolly et al U.S. Pat. No. 4,840,728. We have found, however, that when a vibrator bar having an elastomeric upper contact surface is placed directly under the support rods of a metal screen panel, that there is substantial abrasion produced which can cause the screen to fail long before the width of the flow slots has been increased to an unacceptable dimension by the passage of the abrasive material through them. In addition to the aforementioned patents, Wright, Jr., et al U.S. Pat. No. 3,029,946 shows a resilient mount for a clamp while Herren et al U.S. Pat. No. 4,735,712 shows laminating a layer of elastomeric material to a metal member to shield the metal member from abrasion.